Transmission control in signaling systems



N. C. NORMAN TRANSMISSION CONTROL IN- SIGNALING SYSTEMS May 28, 1935.

2 Sheets-Shet 1 Filed Dec. 27, 1933 IN VE N TOR /v. c. NORMAN ATTORNEY May 28, 1935. N. c. NORMAN TRANSMISSIQN'CQNTROL 'IN SIGNALING SYSTEMS Filed Dec. 27, 1955 2. Sheets-Sheet 2 wvewro/v N. C. NORMAN A TTORNEV Patented May 28, 1935 UNl'iED STATES TRANSMISSION CONTROL IN SIGNALING;

SYSTEMS Nathaniel 0. Norman, New York, N. Y., assignor" to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 27, 1933, Serial nautical 4 Claims. -(Cl .1 79-170) The invention relates to two-way signaling 'sys-. tems and particularly to circuits for controlling transmission in saidsystems.

An object of the invention is to improve the 5 operation of long two-way signaling systems employing signal-controlled switching circuits for suppressing echoes and preventingsinging.

To obtain proper operation of long two-way signaling systems, for example, long four-wire toll telephone circuits, it has been found desirable in the past to utilize in connection therewith signal-controlled switching apparatus for effectively disabling one of the associated twowire paths while signal transmission is taking portion of the signal current from one path and utilizing it to cause the oppositely directed path to be open-circuited or short-circuited, or to in,-

sert a large loss in the oppositely-directedcpath by properly controlling the operation characteristics of an amplifier therein.

In certain circuits of the prior art, these sigrial-controlled devices, or echo suppressors as they are commonlycalled, one of which isassociated with each two-wire path, are located at some intermediate station, for example, at a midway point. the two units of the echo suppressor are located respectively at or near the terminals of the fourwire circuit.

In accordance with the present invention, in two-way signaling systems of the above described types, transmission lockouts are minimized and breakrins are facilitated by making theopera- 40? tion of the echosuppressors depend on the relative amplitude level of waves in the two onewayamplifyingpaths. In one embodiment applicable to a four-Wire telephone system employing terminal echo suppressors, at each terminal waves from the incoming and outgoing one-way paths are fed into a balance circuit which conplace over the other, so as to prevent echo curamplifier-rectifier control circuit, for diverting a In other circuits of the prior art due to said speech currents, the state of balance in the balancelcircuit at thenearterminal is such as to allow normal operation of-therecho suppressor thereat tosuppress the echo path.- Now f if the subscriber,associatedwith the near ,ter: minal startsto talk, a portion of his speech currents will be impressed on the balancercircuit at.- that terminal-along. with; the echo currents-of: thefirst subscriber? and 'will so change the balance of thebalance circuit as to disable the echo' suppressor. Thus, the outgoi g path will become operative to transmit the second subscribers speech'currents to the iirstsubscriber so that the latter will know the former is trying to break in. A singing loss; element is normally included in the output of each one-waypath and is removed by operation of the echosuppre'ssors, As long as both subscribers continue to talk at the same time, thereforepboth suppressors will be disabled andeach subscriber will hear the speech of-the other attenuated by the singing loss. As soon as one subscriber stops to listen, the balance of the balance circuit at his terminal is restored and the circuit functions normally again.

i The objects and ''advantages of the invention will be betterunderstood from the following de- H tailed description, thereof when read in connection with the accompanying drawings in which Fig. l shows schematically a iour-wire toll telephone circuit equipped with transmission control. circuits in accordance with the invention; 7

- Fig, 2 shows ingreater detaila preferred construction of the transmission control circuits utilized in the system otFig. 1;. V i Y The diagram of Fig. 1 is'not an actual circuit diagrambut rather a single-line layout, each line indicating a transmission path. A normal break in a path is indicated byseparate'd arrow heads; agnormal make bypontacting arrowfheads. v To showthat a transmission element is to .be'controlled by a device an arrowpoints from the device towards the element. An arrow directed at a make point indicates that the path'willbe disabled at that point by operation of the control device and an arrow directedgat a break point, indioates'that the break in the path will be eliminated :by operation of the control device. --A resistance element; in a path indicates a finiteloss in. the path; A resistance element shown bridged between the break pointsin path indicates that the path is not disabled but that there is a finite lossnormally in the path which is efiectivelyeliminated when the control device operatesto eliminate the break, s V v The four wir e telephone circuit of Fig. 1 comprises a one-way transmission path'EA includcontacts 4 in shunt with the loss.

2 ing the one-way amplifying devices A1 and A2 for repeating telephonic currents in the direction from west to east between the west two-way line section Lw and the east two-way line section LE, and the one-way transmission path WA' including the one-way amplifying devices A3 and A4 for repeating telephonic currents in the direction from east to west between the east two-way line section LE and the west two-way line section Lwa The oppositely directed one-way repeating paths EA and WA may be connected in substantially conjugate relation with each other ceived from WA to disable the path EA near its input by opening the normally closed contacts 2 therein, and substantially simultaneously to remove eifectively the singing loss 3 from the output'of WA" by closing the normally open switch I 7 Similarly, connected across the path EA near the east terminal of the four-wire circuit is the input link 5 of the echo and singing suppressor S2 adapted to respond 'to telephone signals from EA to disable the path WA near its input by opening the normally closed contacts 6 therein, and substantially simultaneously to remove effectively thesinging loss Tfrom the output of the path EA by closing the normally closed contacts 8 in shunt with the loss. The echo and singing'suppressors S1 and S2 may be of any of the well known types.

Bridged respectively across the output of the path WA at a point near the input to theamplifier A4 and across the input of the path EA at a point near the input to the amplifier A1 are the input links 9 and ID to the'balancing mechanism BMl which is adapted to operate automatically in response to applied waves of a single frequency f1, preferably outside the normal range of the telephone signals transmitted over the paths WA and EA, to'continuously adjust the potentiometer II in link9 until the amount of signal energy entering thebalancing mechanism BM1 from the path WA over link 9 is very nearly equal to the amount of echo energy due to the signals in WA entering BMr from the path EA over link Ill. r Y I Bridged respectively across the link 9 inthe output of the potentiometer H and across the input of the link ID are the input links I? and [3 to the balanced disabler BD1. -The balanced disabler BD1 is adapted to operate in response-to signal currents entering the link [2 by way of than that received by [the link [3' through link 9 from thepath WA. 7

Bridged respectively across the output of the path EA at apoint in the input to'the amplifier Az and across the input 'of'the path WA at a point in the input to the amplifier A3 are the in-.

put links l5 and I6 to the balancing mechanism 'BM2' which is adapted to operate in response to applied waves of a single frequency 12, preferably outside the'normal range of the signals transmitted over the paths WA and EA, to continuously adjust the potentiometer I l in the link [5 until the amount of signal energy entering,

the balancing mechanism BMz from the path EA over link I5 is very nearly equal to the amount "of echo energy due to the signals in EA entering BMz from the path WA over link l6.

Bridged respectively across the link 15 in the output of potentiometer I1 and across the input of link l6 are the input links l8 and 9 to the balanced disabler BDz. The balanced disabler BDz is adapted to operate in response to the signal currents entering link H! by way of link I 6 received by the link l8 through link Hi from the path EA.

A tone source 2| is adapted to be connected across the input of the path WA at a point in the output of the 'echo suppressor contacts 6 therein, by means of the key 22, to send out over the path WA a tone of the frequency f1 to which the balancing mechanism BMiat the west terminal of the four-wire circuit is responsive.

A tone source 23 is adapted to be connected across the input of the path EA at a point in the output of the echo suppressor contacts 2 therein by means of the key 24, to send out over the path EA a tone of the frequency f2 to which the balancing mechanism BMz at the east terminal is responsive.

The preferred construction of the portion of the control circuits schematically shown within the dot-dash box 25 or 25 in Fig. 1 is shown in Fig. 2.

As indicated in Fig. 2, the balancing mechanism BM1 consists of amotor driven device for controlling the setting of the adjustable loss pad H and a control circuit for controlling its operation.

The loss pad 1 l in the input link 9 comprises a potentiometer consisting of a resistance 26 connected across the link 9, and an adjustable arm 21 contacting therewith and which is adapted to be rotated either in a forward or reverse direction in response to rotation of the shaft 28 on which it is mounted, to control the setting of the potentiometer and accordingly the attenuation in the link 9. Two discs, 29 and 30, are suitably mounted on the shaft 28 so as to be rotatable therewith. Thediscs 29 and 39 are adapted to be selectively brought into engagement with the constantly rotating disc 3| driven by the motor 32, under control of the .electromagnets 33 and 34, respectively.

The control circuit of the balancing mechanism BM1 includes a differential relay 35 with the two equal magnetically opposed windings 3S and 31. The winding 35 is adapted. to be energized by the output current of the vaouum'tube rectifier 38 the input of which is coupled through a selective circuit 39 adapted to select the frequency f1, to the link 9 in theoutput of the loss pad H. The Winding 31 is adapted to be energized by the output current of the vacuum tube rectifier 40, substantially identical with the rectifier 38, the input of which is coupled through the selective circuit M; identical with the selective circuit 39, to the 19. The differential relay 35 is equipped with contacts 42 which when closed completes an energizing circuit for the electromagnet 33 from the battery 43, and with contacts 44 which when closed complete an energizing cir cuit'for the electromagnet 34 from the battery 43. I The balanced disabler BDl comprises a differential relay 4% with two equal and magnetically opposed windings 45 and 4'2. The. winding 46 is adapted to be energized by the output current of the vacuum tube rectifier 48 the input of which islcoupledthrough the amplifier 43 in the link l3 across the link 9 inthe output of losspad H, The winding 4? is adapted to-be energized by the output currentof the vacuum tube rectifier B the input of which is coupled through the amplifier 5! in the link l2 to thef link It! in front of the selective circuit4i. 45 isequipped with normally closed contacts !4 whichareadapted to be opened to break the circult of the input link '1; when the magnetic force oi winding 4'! predominates over .that iofxthe winding 46 to the requireddegree in response to a suitable diiierencebetween the amplitude level of thefenergizing currents supplied to the windings 4'6 and 4i. 7 l

The control circuits within the dot-dash portion 25' near the east terminal of the four-wire circuit are the same as shown in 2 except that the seiective circuits in BMz corresponding to the circuits 3% and 4! in BMi' would be deinstead of ii.

The operation of the system will now be de-' scribed by reference to Figs. 1 and 2. It will be assumed first that no speech waves are being received over the two-wire lines Lw and LE by the one-way transmission paths EA and WA. A tone of the frequency f1 is then sent over path WA by connecting the tone source 2! to WA by means of key 22. At thewest terminal avportion of the tone energy is diverted into the link i and operates the echo and singing suppressor S1 provided contacts M are closed. A second portion of the tone energy is transmitted through the singing loss 3 in the path WA provided contacts 4 are open. 7 Part of this latter portion is diverted into the link 9 and is attenuated by potentiometer H therein and the attenuated tone in the output of potentiometer H divides. One part is selected by the selective circuit 39' and is rectified by the vacuum tube rectifier i3 causing'the winding 46 of the differential relay 45 in the balanced disabler HBD]. to beenergized. A' third portion of the tone energy of frequency {1 is amplified by the amplifier A4 in the path WA and'passes outof the four-wire circuit through the hybrid coil connection to the two-wire line 'Lw and returns in the form of an echo to the path EA A portion of the echo energy will be diverted into link l0 from'the path EA and will be divided'between the balance mechanismdillfh and the balanced disabler BB1. One part is selected by the selective circuit 45 and is rectified by the vacuum tube rectifier 48' causing the winding 3? of the differential relay 35 in the halancing'niechanism BMi The differential relay 7 If the amount Of-tOIlB energy entering the vacuum tube rectifier 38 in link 9 is greater than a that entering the vacuum-tube rectifier 49 in link ill, the magnetic force of winding 35 will predominate over the opposing magnetic force of winding 3'! and the relay contacts'42 will close,

directionto increase the attenuation in the link 7 9. p This will continue until equality is established. between the magnetic forces of the windings 36 and 31 of the differential relay 35 whereupon the relay contacts 42 will be opened causing the energizing current to be removed from electromagnet 33 and the connection between the motor and the potentiometer-H to be broken.

It the amount of tone energy entering the vacuum tube rectifier 48 is greaterthan that entering the vacuum .tube rectifier -33, the magnetic force exerted by winding 37 will predominate over that exerted by windingtfi and relay contacts 44 will close, causing battery 43 to energize the electromagnet 34. The electromagnet 34 will then bring disc '30 into engagement with the continuously rotating disc 3|, thereby rotating the arm 27 of the potentiometer H in a clockwise direction and decreasing the attenuation in the link 9 until equality is'established between the magnetic forces of'the windings 36 and 31 of the differential relay 35. The relay contacts 44 will thenlreturn to the open condition causing relay 34 to be deenergi zed and thus the adjustis opened, thereby disconnecting the tone sources 2i from the path WA. 1

Meanwhile, the balance mechanism BM2 and the control circuitstherefor at the east terminal may be adjusted by a tone or" a single frequency f2 sent over the path EA from the tone source 23 in a'manner similar to that just described for the corresponding apparatus at the west terminal of the four-wire circuit;

The apparatus ,at the two terminals of the system is now in adjustment for proper operation of the echo and singing suppressor for signal transmission in eitherdirection.

It will now be assumed that speech waves for transmission from east to west are received over the two-wire line LE and areimpressed through the hybrid coil or other coupling on the transmission path-WA at the east terminal of the four-wire circuit} and at that time no, speech waves have as yet beenreceived at the west terminal from the two-wire line Lw for transmission from west to east over the path EA.

The received speech waves will be amplified by the amplifier A3 and transmitted over the path WA toward the west terminal of the fourwire circuit. At the west terminal a portion of the speech wave energy will be divertedinto the link I, and, the contacts l4 therein being closed will cause operation of the'suppressor S1 thereat, simultaneously closing the contacts 4 to remove effectively the singing loss 3 from the output of the path WA, and opening the contacts 2 in the input of the path EA thereby preventing transmission of echoes dueto the westto east speech currents over the path EA. A second portion of a ill diverted into the link 9 where it will be attenuated by the potentiometer I I. This portion will be blocked from the input to the balancing mechanism BM1 by the selective circuit 39, and will enter the input link I3 of the balanced disabler BD1, where it will be amplified by amplifier 49, rectified by the vacuum tube rectifier 48 and will cause the winding 46 of the differential relay 45' to be energized. A third portion of the speech wave energy, after passing out of the four-wire circuit into the two-wire circuit Lw through the hybrid coil connection will return to the fourwire circuit in the form of echo and enter the path EA. A portion of the speech echowill be diverted into the link I and, being blocked from the input to the balancing mechanism BMi by the selective circuit therein, will enter the. input link I2 to the balanced disabler BD1. The echo currents in link I2 will be amplified by amplifier and rectified by the vacuum tube rec-'- tifier 58 causing the winding 41 of the diiferential relay 45 to be energized.

Because of the previously described adjustment of the balancing mechanism BM1 in response to the tone f1 from the tone source 2!, the magnetic forces due to the rectified speech waves in the windings 46 and 41 of the differential relay 45 will in effect cancel each other and the contacts I4 will remain in the normal closed position; Thus, the echo and singing suppressor S1 will remain connected to the path WA and will function in the usual manner in response to the speech waves received over the path WA and diverted into the link I near thewest terminal to suppress echoes in the path EA and to remove the singing loss from the path WA.

Now, let it be assumed that there is an appreciable pause in the speech waves received over the two-wire line LE and that during the pause speech waves from a west subscriber are received over the two-wire line Lw and are impressed on the input of the transmission path EA at the west terminal of the four-wire circuit.

A portion of the received speech energy will be diverted from EA into the link I0 and, being blocked from the balancing mechanism BM1 by the selective circuit 4|, will enter thejlink I2 of the balanced disabledBDi. In the link I2, the

speech energy will be amplified by the amplifier 5i and rectified by the vacuum tube rectifier 50 causing the winding 41 of the differential relay 45 to be energized. Since the amplifier A4 prevents any portion of the speech wave energy received over the two-wire line Lw from reaching the link 9, no magnetic force is' dev eloped in the winding 46 of the differential relay 45 in the balanced disabler BD1'. Therefore the magnetic force in'winding 41 of the relay 45 will cause the contactsl4 in link I to open, disabling the suppressor S1.

The remaining. portion of the speech waves received from the two-wire line Lw will be amplified by the amplifier Al and sent over the'path EA in the direction from west to east. A portion will be diverted at the east terminal into the link 5 and cause operation of. the suppressor, S2 to simultaneously open contacts 6 to disable WA from transmitting echoes of the west subscibers speech to theeast subscriber, and to close contacts 8 in EA efiectively removing the singing loss '1 from that path. Another portion of the.

speech waves in EA'will be diverted at the east terminal into the link I5 and, being blocked from the balancing mechanism BMz by the selective circuitin the input thereof,- will enter the input link I8 to the balanced disabler BDz. Another portion of the speech currentswill pass from the output of EA into the two-wire line LE, return to WA in the' form of echo currents, and will be diverted into link I6. This portion also will be blocked from the balancing mechanismBMz by the selective circuit in its input, but will enter the other input link I9 of the balanced disabler BDz. Because of the previous adjustment of the balancing mechanism BMz in response to the single frequency tone ,fz transmitted over EA, the magnetic forces resulting from the two inputs to the balanced disabler BDz effectively cancel each other, and the echo and singing suppressor S2 will remain connected to the path EA through the closed contacts 20 in the link 5.

It will now be asumed that speech waves for transmission from east to west are received from the subscriber associated with the two-wire line LE and are impressed on the input of the transmission path WA, and that at first no speech waves are being received from the west subscriber at the west terminal over the two-wire line Lw, so thati the east subscriber gets control ,of the switching circuits at both ends of the system in the manner which has been described. Then let itbe assumed that after an interval and while the east subscriber is still talking, the west subscriber starts to talk. The west subscribers speechwaves will be diverted at the west terminal into" the link I0 and thusinto link I2 where after amplification and rectification it will be supplied to the winding 41 of the differential relay 45 in the balanced disabler BD1 where it will add to the magnetic force of that winding which is already energized by the echo currents due to the east subscribers speech waves. Since the amplifier A4 prevents any portion of wests speech wave energy received over the two-wire line Lw from reaching the link I connected to the path WA, no additional magnetic force-is developed in the winding 46 of the differential relay 45 in the balanced disabler BD1; therefore contacts I4 are broken under control of the superior magnetic force of winding 47, and the echo and singing suppressor S1 will be disconnected from the link I. The echo suppressor contacts 2 in the path EA will then return to the closed condition allowing Wests speech current, as well as easts echo currents to be transmitted over that path to the east terminal, and contacts 4 in WA will return to the open condition l e-inserting the singing loss 3 in that path.

When wests speech waves and easts echo currents transmitted over the path EAreach the east terminal the portionof the energy diverted into link 5 cannot cause operation of the suppressor S2 because the balanced disabler BDz has previously operated under control of easts speech waves'to disable that suppressor by opening contacts 2B in the link 5. The singing loss I there fore is efiective in the output of path EA. This loss together with the equal loss 3 efiective in the output of path WA is suflicient. to prevent the gain around the four-wire loop from exceeding zero decibels and thus prevents singing. Wests speech waves and easts echo currents encounter an attenuation in EA identical with that encountered by easts speech currents and Wests echo currents in WA. When Wests speech waves and easts echo currents reach the link I5, a portion of their energy is diverted and enters the balanced disabler BDz in which its effect is exactly ofiset by the waves entering the balanced disabler BDz through the, link lfi which comprise echoes of wests speec h waves and echoes of easts echo waves.

' 'Thus, with the circuits of the invention,when-' ever both subscribers talk at once and speech wavesare being received in. the four-wire circuit simultaneously from the-subscribers associated with the two-wire lines Lw and LE, the suppressors at both terminals are disabled and speech waves and accompanying echoes are transmitted simultaneously in both one-way transmission paths EA and WA, attenuated only sufiiciently to prevent singing of the four-wire circuit. When speech waves are received over only one of the two-wire lines at a time, operation of the circuit is similar to that of any four-wire circuit equipped with standard types of terminal echo and sing ing suppressors. It follows that two subscribers associated with the terminals of a four-wire circuit equipped in accordance with the invention can both talk at once without either locking the other out, irrespective of the relative starting times of either subscribers speech waves. Thus, either subscriber can let the other know that the former wants to break in. e

In the preferred form of the invention which has been illustrated and described, the tone sources utilized for adjusting the balancing mechanisms are shown as connectable to the oneway transmission paths at the distant ends of the.

paths from the mechanisms which they control. It is also within the scope of the invention to have the tone sources connectable to points at the near ends of the one-way paths, for example, in WA near the point of connection'of link I or 9 thereto, and in EA near the point of connection of link 5 or I5 thereto. Also it would be within the scope of the invention to have the tone sources connected to the one-way transmission paths automatically instead of by keys, for example, by suitable connections to extra leaves on the terminal jacks of the four-wire circuit.

It is also within the scope of the invention to use a vacuum tube vario-losserv of the type described in the copending application of L. F. Smith, Serial No. 676,113, filed June 16, 1933, in place of the motor and magnetic clutch operated potentiometer described herein, or to adjust the potentiometer l l by manual means to obtain equal magnetic forces in the two windings of the difierential relay in BD1 or BD2 when incoming signals are present in the incoming oneway path at a terminal and echoes only of said signals in the outgoing one-way path thereat, in which case the balancing mechanisms BM1 and BM: and associated control circuits and even the tone sources 22 and 23 may be eliminated.

The frequency of the tones utilized to initially adjust the control circuits at the two terminals may be within the frequency range of the transmitted signals if suitable means are provided for preventing false operation by the signals of those frequencies, for example, such as disclosed in the U. S. patent to N. C. Norman No. 1,918,822, issued July 18, 1933, for a similar purpose.

Other modifications of the circuits illustrated and described within the spirit and scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In combination, a four-wire circuit comprising two oppositely directed one-way paths for repeating signals in opposite directions between two two-way circuits, a switching device connected to one of said paths near its output and 5 responsive to signals therein to disable the other one-way path, and control means connected both to said one path near its output and to said other path near its input, which when operated disables said switching device, said control means being adjusted so as to be unoperated whe'ngsignals are impressed thereon from said one path and only echoes of said signals are impressed thereon from said other path, and to operate in response to energy received from said other path when outgoing signals received from the associated two-way circuit are present in the input of said other one-way path, and incoming signals are simultaneously present in the output of said one one-way path.

2. In combination in a two-way signaling system, two two-way lines,.a four-wire repeating circuit comprising two oppositely directed oneway amplifying paths connecting said lines, switching means near each terminal of the fourwire circuit and responsive to signals in the incoming one-way path thereat to disable the outgoing one-way path at that terminal, and control means at each terminal connected both to the incoming and outgoing one-way paths thereat, which when operated disables the switching means at the same terminal, said control means being adjusted to be inoperative when the difference between the energy levels of the outgoing waves in the outgoing one-way path and the incoming Waves in the incoming path at the terminal is such as would be obtained by the presence of incoming signals in the latter path and only outgoing echoes of said signals in the former path, but to be operated when the difference between the energy level of waves in the incoming and outgoing paths at said terminal are such as would be obtained by the presence simultaneously of incoming signals in the incoming path and outgoing signals in the outgoing path.

3. In combination in a two-way signaling sys tem, two two-way lines, a four-wire repeating circuit comprising two oppositely directed oneway repeating paths connecting said lines, a loss sulficient to prevent singing in said four-wire circuit normally connected in each one-way path, a switching device connected to the incoming oneway pathat each terminal of the four-wire circuit, and responsive to incoming signals therein to disable the outgoing one-way path at that terminal and substantially simultaneously to remove efiectively the loss in the one-way path incoming at that terminal, wave-responsive control means at each terminal having its input connected both to the output of the incoming one-way path thereat and to the input of the outgoing oneway path in front of the disabling point therein at the same terminal which when operated disables the switching means at that terminal, said control means being adjusted to be unoperated when the difference between the energy levels of 4. The combination of claim 1 and in which said control means comprises a relay having two equal and oppositely wound windings, means responsive to the incoming Waves in said one oneway path for energizing one of said windings,

in said other winding than in said one winding for effectively disconnecting said switching device from said one one-way path, and means for initially adjusting said control means so that equal energizing currents will be'supplied to the two windings when incoming signals are present in the output of said one one-way path and only outgoing echoes of said incoming signals insaid other one-way path. '7

' NATHANIEL c. NORMAN. 

